This invention relates to a method of making high-strength steel U-Bolts from end-threaded rods that are flattened along their center section. More particularly, it relates to a method in which a flattened end-threaded rod of high strength steel is cold formed into a U-bolt having a desired geometric shape, such that the strength of the U-bolt remains substantially the same or greater than the flattened end-threaded rod.
Many methods exist for the production of high-strength steel parts and other members. In particular, U.S. Pat. Nos. 5,236,520; 5,330,594; 5,496,425; 5,538,566; 5,094,698; 5,704,998; 5,454,888; and 5,453,139 each disclose a method for forming such parts or other members and each is hereby incorporated by reference in its entirety. One such part is a flattened U-bolt.
Many original equipment manufacturers (OEM's) in the automotive and trucking industries currently use flattened U-bolts for a number of reasons. Two of these reasons are to increase the contact area between the axle and the U-bolt, and to increase the clearance between other parts of the suspension assembly and the U-bolt. The increase in the contact area is considered necessary because the OEM's have saved material costs and reduced weight by reducing the thickness of the material used in their axles. With the change to such thinner and lighter weight axles, a non-flattened U-bolt has the potential to collapse the axle. The contact surface area between the non-flattened U-bolt and the axle is limited to the rounded, circumferential portion of the U-bolt thereby concentrating the applied load in a smaller area of the axle and increasing the possibility of deformation or failure of the axle. Therefore, flattened U-bolts are preferred because the flattened portion increases the surface area of contact between the U-bolt and axle, thereby distributing the load and decreasing the likelihood of damaging the axle.
Vehicles using these flattened U-bolts are often modified after they leave the factory. The modification typically requires a replacement U-bolt. Suspensions are frequently modified to meet customer requirements or to increase the load carrying capabilities of the vehicle. In most modifications, one or more springs are added to the existing suspension and a longer U-bolt is required to secure the suspension.
Known sources for supplying replacement U-bolts cannot produce a flattened U-bolt. Typically, a spring and suspension repair facility will have the equipment and material to cold bend a high-strength replacement U-bolt. Such facilities cold bend high-strength, end-threaded rods into U-bolts that will meet the length requirements of the repair or modification.
However, the spring and suspension repair facility does not have access to an inventory of flattened high-strength steel, flattened end-threaded rods. Therefore, when a vehicle that has an OEM flattened U-bolt is modified, and longer U-bolts are required as a result of the modification, the replacement U-bolt is currently made from non-flattened material. The resulting non-flattened, cold bent, U-bolts do not meet the specifications of the OEM design and have the possibility of damaging the axle.
Flattened U-bolts are currently produced by warm forming. In warm forming, a high-strength steel blank or rod is heated to a temperature that is below the recrystallization temperature of the steel and low enough that the strength of the blank or rod is not lowered, but high enough to enhance forming. Referring to FIGS. 2A-2C, the high-strength steel blank or rod 11 has its opposite ends 13 threaded and is flattened and bent into a U-bolt 15 using a one stroke or two stroke operation.
In the current one stroke operation, a flat 17 is initially formed on the rod or blank 11 thereby generating heat and warming the flattened or central portion 17 of the rod or blank 11. Immediately thereafter in the remainder of the machine stroke, the flattened and warmed rod 11 is bent into the U shape. In the current two stroke operation, the end-threaded 13 blank or rod 11 of high-strength steel is flattened with the first stroke and then, on a separate tool or machine, the flattened rod 11 is bent with a second stroke immediately (i.e., within 3-10 seconds) after being flattened in the first stroke.
The flattening portion of both the one and the two stroke operations causes the flattened section 17 of the blank or rod 11 to increase in temperature. This temperature increase in the one or two stroke operation enhances the formability of the flattened section 17 of the rod 11. Typically, the flattening step of the one or the two stroke operation raises the temperature of the rod 11, at least in the flattened section 17, from ambient to about 250° F. Without the temperature increase that occurs during the flattening operation, prior to this invention it was not possible to form U-bolts 15 from flattened rods or blanks 11 because cold bending the flattened end-threaded rod 11 commonly resulted in the rod breaking.
Therefore, a spring and suspension repair facility, while having the equipment to cold bend a high-strength replacement U-bolt, does not have the resources to or advantages of warm forming a rod or blank into a U-bolt. Due to the significant change in geometry of the flattened end-threaded rod relative to the non-flattened end-threaded rod, the flattened rods commonly break, crack or fracture when being cold formed into a U-bolt. Therefore, the spring and suspension repair facility simply cold forms a non-flattened end-threaded rod for use in the modified suspension system in contravention of the OEM's specifications. The industry needs a solution to this problem of the inability to consistently and reliably cold form a flattened end-threaded rod into a flattened U-bolt.